Lucario is not a Legendary in the Game, BUT he becomes a Legendary when he is mentioned in the movie "Lucario and the mystery's of Mew" As the Aura Guardian Lucario. This is when he masters Aura sphere. So, is he a Legendary? Sometimes, If your lucario masters aura sphere and is a high level, then technically he is a Legendary.

has anyone noticed that lucario is an EVOLED Pokemon. look up Rilou. honestly has no one notice that. LEGENDARY POKEMON CANT BE EVLOVED. it may be a bad ass pokemon, also one of the coolest, but it isnt a legendary.

Well, if you're going for the movie, there's only one. However in the games, there's way more. Then again, there's more than one Cresselia in D/P so.... The point I'm trying to make is I don't think so, since in the games there's more than one, they can be different genders, and they evolve from Riolu.